The COP are closed headphones while the DT990 are open so the COP has more bass then the DT990 but suprisingly not by much. The mid bass is similiar, strong in both of them but the sub bass is better noticable on the COP. Even though the DT990 are open (most open cans have weak bass IMO) these cans can give you a lot of bass for what they are. Of course it depends on what you're gonna use them for.
For games and movies these are excelent, the open soundstage plus the clean sound that they have mixed with their strong mid bass is a great combo for immersive gaming. Some may say closed headphones are better for immersive gaming due to the stronger bass they present but IMO there's nothing better than playing something like a horror game or an RPG and being in a dungeon hearing sounds from diffrent directions and feeling like you're actually in the cave
Now if you're gonna use them mostly for music, they can be a hard choice due to the treble spike they have. EQ them will help a lot in that aspect. To be honest for classical music or things like that they are great but if you're more into electronic music and bassheavy things (since you're a basshead) you'll probably want something else. As I'm a basshead aswell I would recomend the SZ series since these have 55mm carbon diagram units JUST for bass. The SZ's are pretty much the high end for bass and cost around the same as the DT's but they are closed so forget about the soundstage. They also have a dark sound so I would recomend them for bass use ONLY.
Now about amping, they both NEED an amp, specially the DT990. So if you're not gonna get an amp than cross these off the list. If you're gonna get an amp for the DT990 I would recomend a tube amp since they help a lot with the treble spike and give a warmer sound making the experience even better. Along the amps I tested with these cans I would recomend the Schiit Vali. Good price for a tube amp with this kind of power. Fiio amps suprisingly work well too with the DT but I would still go with a tube amp. On the other hand don't go with bright amps like the Schiit Modi since these will make the DT's treble spike worse. The O2 amp is also a good choice although the 02 is a neutral solid state amp so no change in sound, only good for amping purposes.
In short if you're a cronic basshead and just want new headphones for bass I would go for something else, preferably closed headphones like the ones I recomended or something else around your budget, if not and want something that does a lot of things good like games and movies than these are great!
If you need info about anything else just ask
But I still need a portable amp, what do you guys recommend as a good entry level protable amp to pair the DT990 with? Would the FiiO E11K be suitable? or would that be skimping out a little to much? 
